1. Field of the Invention
This invention relates to a focus control apparatus, more particularly to a focus control apparatus for controlling an optimized light convergence point of light beam relative to a specified recording surface of a recording medium, which is irradiated onto the recording surface of the recording medium.
2. Description of the Related Art
Nowadays, information recording mediums of capable of recording a large quantity of information with a high density called a digital video disk or a digital versatile disk (designated as DVD hereinafter) and systems using the information recording mediums are about to be widely used. For example, there is one type of disk comprising two information recording surfaces formed in two layers, respectively, which interpose an interposing layer (a spacer or a space region) between them. In order to read out information from one surface of such a type of disk with an optical pickup, a light beam must be converged on a desired information recording surface in one of the layers.
A focus jump operation for moving a focus of the light beam is performed from one information recording surface in one layer to the other information recording surface in the other layer on the basis of the detection of a zero crossing point in a focus error signal which is generated based on an output of the pickup.
More specifically, the pickup has a focus actuator. The focus actuator drives an objective lens having a predetermined focus of an optical system in an optical axis direction, which produces a light emission as the light beam, to move the focus of the light beam in a direction perpendicular to the disk surface. At an initial stage of the focus jump operation, an acceleration signal is supplied to the focus actuator as a focus jump starting signal for moving the focal point of the light beam to a target recording surface. Then, a series of the following operations are conducted in accordance with zero crossing points sequentially detected from the focus error signal acquired during the movement of the focus actuator in response to the acceleration signal; termination of the supply of the acceleration signal, supply of a deceleration signal to the focus actuator for stopping the movement of the focus actuator in response to the acceleration signal, and restart of a focus servo for the target recording surface by the completion of the supply of the deceleration signal.
FIG. 1 shows a position of the focal point P of the light beam in the internal layer of the disk which is determined by the objective lens 100, a focus error signal acquired when the focal point P moves in the optical axis direction, and a relationship therebetween. In FIG. 1, it is found that the focus error signal fundamentally draws an S-shaped profile symmetrical with a zero level point (zero crossing points ZC00 and ZC10). It is noted that the zero level point indicates that the focal point P exists on the recording surface. A range between minimum and maximum values of one S-shaped profile generally corresponds to a controllable range of a focus servo loop. The controls for the supplies of the above acceleration and deceleration signals are performed on a way of the movement of the focal point from one recording surface to the other recording surface, so that they are performed based at a timing on which one of the zero crossing points ZC00, ZC01, ZC1-1 and ZC10 is detected.
However, the focus error signal acquired during the movement of the focal point between these recording surfaces is influenced by defects of the surface of the disk, the recording surface thereof or defects of the surfaces of other internal layers. The focus error signal is also influenced by incorrectly formed marks equivalent to the above defects (designated as defects hereinafter) or a crosstalk. More specifically, if the light beam scans the defects or the crosstalk occurs, the level of the focus error signal significantly drops or rises abruptly. The focus error signal may show a false zero crossing point different from a desired zero crossing point produced by the S-shaped profile as shown in FIG. 1. In this case, it is impossible to implement the correct supply of the foregoing acceleration and deceleration signals. Therefore, the focus actuator operates erroneously, leading to a failure in the focus jump operation.
Taking the foregoing problems in consideration, an object of the present invention is to provide a focus control apparatus which is capable of successfully performing a proper focus jump operation without an erroneous operation of a focus actuator.